I recently replaced my slider header with my slide out in place. I am going to try to summarize what I did here, because I didn't see anyone providing instructions anywhere, and it took a lot of work and help from friends.
First step, remove interior panelling - pretty straightforward. I also had to remove my bedroom wall.
Next, use a jack to support the ceiling. I used a piece of plywood, and a chunk of timber, supported by a tall jack, with the jack far enough back that the slide out would come part of the way in before it hit.
Next, cut out around the header. I used a multitool with a carbide tipped blade. You have to cut out the screws and the glue that holds it in place. My slideout also had a piece with a bunch of screws into the header outside of the trailer. The screws were covered by a rubber seal. A sawzall might have been quicker and better, but I didn't have one. This took a long time, and was hard to do. I then cut the header into about 18" lengths and removed them one at a time. You have to be careful not to cut or push to hard against the aluminum siding.
I made a new header with three strips of 1/2" plywood screwed and glued together with construction adhesive, and I angle ground off the screws (I put them all the way through, as I was advised that the tips of the screws don't really have any holding power.
I passed the new header through the bedroom window, which I had removed to fix some water damage.
I screwed the new header in place, attaching the weather stripping on the bottom first. I had to cut the corners off the weather stripping to get it in place. It was built to go on from the outside. Replace all of the screws from outside. There is no way to really get at screwing the new header to the strip of wood above, so I used galvanized rectangular joiners - I can't remember what they are called, and lots of nails. This is essential, otherwise the header will pivot when you use the slide.
I fixed the wall parts that were damaged, and closed it up with new panelling. If anyone has any questions I will help if I get notifications - I've never posted here before, so I'm not sure if I will.